Disposable absorbent products, such as disposable diapers, sanitary napkins, and the like, typically include a liquid impermeable outer covering, an absorbent layer, and an inner layer which contacts the skin of the wearer. The inner layer is referred to in the art as coverstock, topsheet, or, in diaper applications, diaper liners.
Typically, the liner or coverstock layer is a liquid permeable, porous nonwoven fabric, such as a carded web or a spunbonded web. The absorbent core can include wood pulp fibers and optionally superabsorbent particles designed to absorb many times their own weight in liquid. To provide a comfortable yet effective product, the liner layer ideally permits liquid to flow through it rapidly into the absorbent layer ("rapid strike through") but does not permit or, at a minimum does not facilitate, re-transmission of liquid from the absorbent layer to the "wearer" side of said inner layer ("resists rewet").
Many conventional absorbent products, however, do not provide adequate liquid strike through and rewet properties. Often liquid is delivered to the absorbent system at a rate faster than the rate at which the system can transport the liquid away from the wearer's skin into the absorbent core (i.e., the liquid uptake rate of the system is slower than the liquid delivery rate). Even if the absorbent system can adequately handle an initial liquid surge, once wetted, the absorbent material can lose resiliency and collapse, thus affecting the ability of the system to handle subsequent liquid surges. As a result, at least a portion of the liquid can pool on the liner layer and/or wick back to the liner layer adjacent the wearer's skin. This can cause wearer discomfort, skin irritation, and leakage.
Design features, such as elastic gathers at the leg and waist region of the product, can alleviate leakage. Alterations to the design and/or amount of the absorbent material can also reduce these problems. While these and other design modifications can minimize leakage, they do not alleviate these problems, and can in some cases aggravate the problem.
Additional layers ("surge" layers) can be introduced between the absorbent layer and the liner layer to improve fluid uptake and to reduce wetback. Typically, surge layers are relatively thick, high loft carded nonwoven webs bonded using through-air bonding techniques. Through-air bonding bonds the fibers of the fabric together to provide fabric integrity while also maintaining the desired fabric structure (such as loft, density, etc.) to provide the desired liquid transport properties. Exemplary through-air bonded nonwoven webs useful as surge layers in absorbent personal care products are described in U.S. Pat. No. 5,486,166 to Bishop et al., U.S. Pat. No. 5,490,846 to Ellis et al., and U.S. Pat. No. 5,522,810 to Allen, Jr. et al. These patents recognize that through-air bonding should be used to control the structure of the fabric (such as level of compression or collapse of the structure).
While these and other fabrics can be effective, it can be difficult to provide through-air bonded fabrics which have adequate liquid transport properties yet also have desired strength, bulkiness, compressibility, resilience, and the like. Further, manufacturing through-air bonded carded fabrics can be slow, and the fabrics can be expensive relative to other types of nonwoven fabrics. Still further, these fabrics are typically provided as roll goods with small fabric yardages per roll, thus increasing shipping costs.
Chemically bonded (i.e., adhesively bonded) carded nonwoven fabrics and calendered carded nonwoven fabrics also can be used as a liquid transport layer in an absorbent product. These fabrics, however, have relatively low loft, and thus the liquid transport properties are not as desirable as those exhibited by through-air bonded carded webs.